Method of Recovering Aqueous N-Methylmorpholine-N-Oxide Solution Used in Production of Lyocell Fiber

ABSTRACT

A method of recovering aqueous N-Methylmorpholine-N-Oxide solution used in production of Lyocell fiber comprises following steps. Bleach means for decoloring coloration in aqueous NMMO solution via alternate blow-mixing adsorption mode and static suspending adsorption mode reiteration. Filtration means for purifying the activated carbon powder and impurities by two filtering stages of first coarse filtering stage and second fine filtering stage. Concentration means for intensifying aqueous NMMO solution to obtain a condensed aqueous solution without NMMO solvent and a concentrated aqueous solution with NMMO solvent respectively by a sequential multi-stage evaporating system. Refinement means for purifying aqueous NMMO solution with promoting purity of concentrated aqueous solution to obtain required recovered aqueous solution by adding suitable agents in the redox reactions involved. Owing to streamlining and simplicity, the method not only has better competitiveness from promoted recovery cost, efficiency and quality but also meets regulations of environmental protection.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation in part of Non provisional application, Ser. No.12/818,912, field Jun. 18, 2010. entitled “Method of recovering aqueousn-methylmorpholine-n-oxide solution used in production of lyocell fiber”

FIELD OF THE PRESENT INVENTION

This invention relates to a recovering method of solvent used inproduction of fiber, more particularly to a recovering method of solventused in production of Lyocell fiber.

BACKGROUND OF THE INVENTION

Lyocell fiber is made from natural cellulose. Consequently, wasteproducts of Lyocell fiber are naturally biodegradable and areeco-friendly without incurring environmental issues. Lyocell fiber hasmechanical strength and tenacity near to that of synthetic fiber.Moreover, Lyocell fiber has excellent draping property, sufficientthickness feeling, comfortable in touch feeling, nice hygroscopicity andeasy in dyeing. Furthermore, Lyocell fiber is easily blend-spinning withother materials of natural or synthetic fibers so that the finalproducts from Lyocell fiber become high performance characteristics andadded-value fabrics due to good quality and easiness in process.

Please refer to FIG. 1. An existing production process of Lyocell fibermainly comprises following four steps: blend, dissolution, spin andrinse, wherein the blend step means for mixing wood pulp and dissolvingsolvent of primary N-methylmorpholine N-oxide (NMMO) to form preliminarymixture; the dissolution step means for dissolving cellulose in thepreliminary mixture to form spinning dope;

the spin step means for spinning and extruding dope out of spinnerets toform raw spinning filaments; and the rinse step means for removingresidual primary NMMO dissolving solvent in the raw spinning filamentsvia washing and drying processes to obtain refined products in Lyocellfibril-filaments of natural cellulose fiber.

Since the primary NMMO dissolving solvent features nontoxic, odorlessand high boiling point, the production process of Lyocell fiber via theprimary NMMO dissolving solvent is more eco-friendly than a conventionalproduction process of synthetic fiber. However, the primary NMMOdissolving solvent is relatively expensive. Therefore, the primary NMMOdissolving solvent is usually recovered and reused so as to reduce theoverall cost in mass production of Lyocell fiber. Though the currentrecovery rate of primary NMMO dissolving solvent from NMMO aqueoussolution for the existing technique is over 99.5% so that the pollutionissues incurred can be effectively obviated, the cost in recoveringprimary NMMO dissolving solvent is not essentially reduced. Accordingly,how to further reduce the processing cost in recovering primary NMMOdissolving solvent from NMMO aqueous solution so as to indirectly lowdown the overall processing cost in production process of Lyocell fibervia the primary NMMO dissolving solvent becomes a urgent and criticalissue.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of recoveringaqueous N-Methylmorpholine-N-Oxide solution used in production ofLyocell fiber so that the recovered aqueous NMMO solution can be reusedrepetitively as well as the recovery efficiency of the NMMO solvent canessentially promoted by the high performance thereof.

The method of recovering aqueous N-Methylmorpholine-N-Oxide solutionused in production of Lyocell fiber of the present invention comprisessteps of bleach (step 1), filtration (step 2), condensation (step 3) andrefinement (step 4), wherein:

The bleach in step 1 means for decoloring the aqueous NMMO solution:

Firstly, the aqueous NMMO solution to be recovered is loaded into ableaching tank, and activated carbon powder featuring with goodadsorbability and suspendability is added into the aqueous NMMO solutiontherein; secondly, the activated carbon powder and the aqueous NMMOsolution are mixed together by using an agitation blower; and finally,the agitation blower is intermittently energized so that an alternateblow-mixing adsorption mode and static suspending adsorption modereiterates to have activated carbon powder fully contacted with theaqueous NMMO solution thoroughly in an energy-efficient manner;

The filtration in step 2 means for purifying the aqueous NMMO solution:

Two filtering stages of first coarse filtering stage and successivesecond fine filtering stage (ultrafiltration UF) are orderly adopted soas to remove the activated carbon powder and impurities from the aqueousNMMO solution 1, which has been decolored in previous bleach process;for the first coarse filtering stage, the activated carbon powder andthe impurities of large particle size appeared in previous bleachprocess can be removed; for the second fine filtering stage(ultrafiltration UF), the tiny impurities of small particle size can beremoved;

The concentration in step 3 means for intensifying the aqueous NMMOsolution:

A sequential multi-stage evaporating system is adopted so as tointensify the aqueous NMMO solution, which has been purified in previousfiltration process so that a condensed aqueous solution without NMMOsolvent and a concentrated aqueous solution with NMMO solvent arerespectively obtained; the sequential multi-stage evaporating systemmainly comprises a first evaporating vessel with a first steam tank, asecond evaporating vessel with a second steam tank and a thirdevaporating vessel with a third steam tank, wherein: the firstevaporating vessel and the first steam tank are connected by a firststeam inlet pipe while the first steam tank and the second evaporatingvessel are connected by a first steam outlet pipe such that the firststeam tank is connected to a first vacuum pump; by controlling theconcentration of the recovered aqueous solution at the outlet of thefirst evaporating vessel in range of 10-20 wt % and the concentration ofthe recovered aqueous solution at the outlet of the second evaporatingvessel in range of 22-38 wt % as well as feeding the steam evaporated bythe recovered aqueous solution at the outlet of the third evaporatingvessel back to the first evaporating vessel as supplementary steamsource via the steam recovering pipe after it has been orderly processedby the third steam tank, separating tank and steam compressor, theoverall recovered quantity of the concentrated aqueous NMMO solutionunder the same consumed quantity of the primary steam source can besubstantially increased so that the goal of promoting recoveryefficiency can be achieved; similarly, the condensed aqueous solutioncollected by the cold condensed water pipe from the first evaporatingvessel, second evaporating vessel and third evaporating vessel can alsobe recovered for reusing in the rinse process of the Lyocell fiberproduction to remove the solvent and impurities attached on the rawfilaments; and

The refinement in step 4 means for purifying the aqueous NMMO solution:

To perform the refinement step here, an oxidizer (namely oxidizingagent) is added into the concentrated aqueous NMMO solution processed byprevious refinement process (step 3) so that the residualN-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide(NMMO) via oxidation reaction by the oxidizer; after the oxidationreaction aforesaid, some residual oxidizer becomes redundant impurity,which should be completely removed anyhow; accordingly, a reducer(namely neutralizing agent) is added into the concentrated aqueous NMMOsolution processed by previous oxidation reaction process aforesaid toneutralize the residual oxidizer via reduction reaction by the reducerso that a recovered aqueous NMMO solution of high purity is obtained;wherein, the final applied quantities for the oxidizer and the reducerare decided by the testing result of the concentrated aqueous NMMOsolution processed by foregoing redox reaction (namely reductionreaction and oxidation reaction) via potentiometric titration.

In conclusion of disclosure heretofore, the method of recovering aqueousN-Methylmorpholine-N-Oxide solution used in production of Lyocell fiberof the present invention features novelties in following processingsteps: bleach means for decoloring coloration in aqueous NMMO solutionvia alternate blow-mixing adsorption mode and static suspendingadsorption mode reiteration; filtration means for purifying theactivated carbon powder and impurities by two filtering stages of firstcoarse filtering stage and second fine filtering stage; concentrationmeans for intensifying aqueous NMMO solution to obtain a condensedaqueous solution without NMMO solvent and a concentrated aqueoussolution with NMMO solvent respectively by a sequential multi-stageevaporating system; and refinement means for purifying aqueous NMMOsolution with promoting purity of concentrated aqueous solution toobtain required recovered aqueous solution by adding suitable agents inthe redox reactions involved. Therefore, the recovering method of thepresent invention not only has completely recovered massive aqueous NMMOsolution to substantially reduce wastes discharged to the environmentbut also has almost recovered the NMMO solvent in the aqueous NMMOsolution to essentially reduce material cost in the production ofLyocell fiber. Thus, the features of the present invention not only saveprocessing cost but also meet requirements of environment protection.Owing to streamlining and simplicity, the method not only has bettercompetitiveness from promoted recovery cost, efficiency and quality butalso meets regulations of environmental protection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a currently existing process forLyocell fiber.

FIG. 2 is a flowchart illustrating a preferred exemplary embodiment in arecovering method of aqueous solution used in production of Lyocellfiber for compatible with the existing process of Lyocell fiber used inthe present invention.

FIG. 3 is a schematic diagram of a preferred exemplary embodiment in ableach step showing the addition of activated carbon powder in ableaching tank in association with a disposition of an agitation blowerfor the present invention.

FIG. 4 is a schematic diagram of a preferred exemplary embodiment in aconcentration step showing the application of a sequential multi-stageevaporating system, which is also known as serial stepwisepressure-descending multi-effect evaporator system, for the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIGS. 2 through 4, which are schematics illustrating apreferred exemplary embodiment in a recovering method of aqueoussolution used in production of Lyocell fiber having massive aqueous NMMOsolution with primary N-methylmorpholine-N-oxide (NMMO) dissolvingsolvent of low concentration for compatible with the existing process ofLyocell fiber used in the present invention. As shown in FIG. 1, theexisting production process of Lyocell fiber mainly comprises four stepsof blend, dissolution, spin and rinse. Wherein, other than the existingprimary NMMO dissolving solvent used in dissolving cellulose, theaqueous NMMO solution also contains a bit of residual N-methylmorpholine(NMM), which is created by the decomposition of heating primary NMMOduring chemical reaction in dissolving cellulose process by the primaryNMMO dissolving solvent.

A preferred exemplary embodiment for the method of recovering aqueousN-methylmorpholine-N-oxide (NMMO) solution used in production of Lyocellfiber comprises following steps: bleach (step 101), filtration (step102), condensation (step 103) and refinement (step 104), wherein:

The bleach in step 101 means for decoloring the aqueous NMMO solution 1.

Generally, the speed of chemical reaction is proportional to thetemperature involved. Therefore, the dissolving efficiency of thedissolving process in production of Lyocell fiber is increased byheating. However, heating also incurs harmful coloration to contaminatepigment impurities into the aqueous NMMO solution 1. Accordingly, bleachprocess to the aqueous NMMO solution 1 becomes necessary in initialstage. Referring to FIG. 3, the aqueous NMMO solution 1 to be recoveredis loaded into a bleaching tank 2, and activated carbon powder 3featuring with good adsorbability and suspendability is added into theaqueous NMMO solution 1. The activated carbon powder 3 and the aqueousNMMO solution 1 are mixed together by using an agitation blower 4. Theagitation blower 4 is intermittently energized so that an alternateblow-mixing adsorption mode and static suspending adsorption modereiterates to have activated carbon powder 3 fully contacted with theaqueous NMMO solution 1 thoroughly in an energy-efficient manner.Thereby, the adsorption efficiency of the activated carbon powder 3 isessentially increased. Wherein, the blow-mixing adsorption mode meansfor blowing ambient air into the bleaching tank 2 by impeller rotationof the agitation blower 4 to have activated carbon powder 3 fullycontacted with the aqueous NMMO solution 1 thoroughly to facilitate thespeed of the bleach process while the static suspending adsorption modemeans for keeping the aqueous NMMO solution 1 in stationary manner tolet the aqueous NMMO solution 1 precipitate automatically to saverelated energy. By means of a timer switch 5 acting on the agitationblower 4, the blow-mixing adsorption mode and static suspendingadsorption mode are intermittently alternated in reiterative fashion sothat overall bleach efficiency is substantially increased. A durationratio for the blow-mixing adsorption mode and static suspendingadsorption mode is in range of 1:3-1:6. Moreover, total time in thebleach process of the aqueous NMMO solution 1 is not longer than 8hours, while the total time in the bleach process of the aqueous NMMOsolution 1 is set to 8 hours in this preferred exemplary embodiment. Bymeans of blow-mixing adsorption mode, not only the activated carbonpowder 3 can be fully contacted with the aqueous NMMO solution 1thoroughly but also the speed of the bleach process can be facilitatedwhile by means of static suspending adsorption mode, not only theprocessing energy can be essentially saved but also the efficiency ofthe bleach process can be enhanced. Preferably, the added dosage of theactivated carbon powder 3 is in range of 0.05 wt %-0.10 wt % on thebasis of total weight for the aqueous NMMO solution 1.

The filtration in step 102 means for purifying the aqueous NMMO solution1.

Two filtering stages of first coarse filtering stage and successivesecond fine filtering stage (ultrafiltration UF) are orderly adopted soas to remove the activated carbon powder 3 and impurities from theaqueous NMMO solution 1, which has been decolored in previous bleachprocess, in this preferred exemplary embodiment. For the first coarsefiltering stage, a filter cartridge having filtering material with poresize in range of 1 μm-100 μm (1 .mu.m-100 .mu.m) is used so that theactivated carbon powder 3 and the impurities of large particle sizeappeared in previous bleach process can be removed. For the second finefiltering stage (ultrafiltration UF), a filter material with pore sizein range of 0.01 μm-1 μm (0.01 .mu.m-1 .mu.m) is used so that the tinyimpurities of small particle size can be removed. After orderlyprocesses of foregoing first coarse filtering stage and successivesecond fine filtering stage (ultrafiltration UF), the cleanness of theaqueous NMMO solution 1 reaches that of a fresh NMMO solvent.

Wherein, a cartridge filter is used in the first coarse filtering stage.Preferably, in order to increase a speed of the coarse filtration, afilter aid is beforehand pre-coated over the surface of the cartridgefilter, and the filter aid is also added into the bleached aqueous NMMOsolution 1 with quantity in range of 0.03-0.05 wt %. Moreover, thefilter aid is made from mixture of diatomaceous earth and cellulose withweight ratio of the diatomaceous earth to the cellulose is 4:1preferably. By means of foregoing double uses of the filter aid, thefirst coarse filtering stage not only prevents the activated carbonpowder 3 from accumulating on the surface thereof in hindering thefiltering speed but also regularly maintains filtering effect of highperformance without decay owing to valid filtering area is increased.

It should be noted that some filtering dregs resulting from the firstcoarse filtering stage could be centrifugally dehydrated aftercompletion of the first coarse filtering stage, wherein the filteringdregs contain residual filter aid and a larger quantity of residualactivated carbon powder 3, which is mostly accumulated on the surfaceportion. After the filtering dregs is scraped off, the residual filteraid therein can be recovered and reused in the first coarse filteringstage step.

The concentration in step 103 means for intensifying the aqueous NMMOsolution 1. A sequential multi-stage evaporating system is adopted so asto intensify the aqueous NMMO solution 1, which has been purified inprevious filtration process, in this preferred exemplary embodiment, sothat a condensed aqueous solution without NMMO solvent and aconcentrated aqueous solution with NMMO solvent are respectivelyobtained. Practically, the sequential multi-stage evaporating system isa serial stepwise pressure-descending multi-effect evaporator system.The evaporating effectiveness of the sequential multi-stage evaporatingsystem depends on the number of stages. In the preferred exemplaryembodiment, the number of stages is 3 to have optimal trade-off betweenthe evaporating effectiveness thereof and facility cost thereof althoughthe number of stages is arbitrarily selected, and the operating mode isin series instead of in parallel. Therefore, the serial stepwisepressure-descending multi-effect evaporator system in the preferredexemplary embodiment is actually a sequential tri-stage evaporatingsystem.

The sequential multi-stage evaporating system mainly comprises a firstevaporating vessel 10 with a first steam tank 11, a second evaporatingvessel 20 with a second steam tank 21 and a third evaporating vessel 30with a third steam tank 31, wherein:

The first evaporating vessel 10 and the first steam tank 11 areconnected by a first steam inlet pipe C1 while the first steam tank 11and the second evaporating vessel 20 are connected by a first steamoutlet pipe 12 such that the first steam tank 11 is connected to a firstvacuum pump 13; Moreover, the first evaporating vessel 10 and secondevaporating vessel 20 are connected by a first solution recovering pipe14, on which a first concentration meter 15 and a first suction pump 16are disposed respectively;

The second evaporating vessel 20 and the second steam tank 21 areconnected by a second steam inlet pipe C2 while the second steam tank 21and the third evaporating vessel 30 are connected by a second steamoutlet pipe 22 such that the second steam tank 21 is connected to asecond vacuum pump 23; Moreover, the second evaporating vessel 20 andthird evaporating vessel 30 are connected by a second solutionrecovering pip 24, on which a second concentration meter 25 and a secondsuction pump 26 are disposed respectively;

The third evaporating vessel 30 and the third steam tank 31 areconnected by a third steam inlet pipe C3 while the third steam tank 31is connected to a third vacuum pump 32 and a third steam outlet pipe 33,which is further connected to a separating tank 40 for steam and aqueoussolution; Moreover, the third evaporating vessel 30 and a second suctionpump 70 are connected by a third solution recovering pipe 34, on which athird concentration meter 35 and a third suction pump 36 are disposedrespectively;

Wherein, the aqueous solution outlet of the separating tank 40 isconnected to the third solution recovering pipe 34 while the steamoutlet of the separating tank 40 is connected to a mechanical steamcompressor 41, which is further connected to a first evaporating vessel10 via a steam recovering pipe 42.

For the first evaporating vessel 10, other than the recovered steambeing fed via the steam recovering pipe 42, a primary steam from a steamboiler (not shown) is supplied via an input pipe for steam 60, and anaqueous NMMO solution 1, which has been purified in previous filtrationprocess, is also supplied by an input pipe for aqueous NMMO solution 50,which orderly passes through the third steam tank 31, second steam tank21 and first evaporating vessel 11 as well as a heat exchanger 6 and ainput pump 51 for aqueous NMMO solution 1.

By controlling the concentration of the recovered aqueous solution atthe outlet of the first evaporating vessel 10 in range of 10-20 wt % andthe concentration of the recovered aqueous solution at the outlet of thesecond evaporating vessel 20 in range of 22-38 wt % as well as feedingthe steam evaporated by the recovered aqueous solution at the outlet ofthe third evaporating vessel 30 back to the first evaporating vessel 10as supplementary steam source via the steam recovering pipe 42 after ithas been orderly processed by the third steam tank 31, separating tank40 and steam compressor 41, the overall recovered quantity of theconcentrated aqueous NMMO solution under the same consumed quantity ofthe primary steam source can be substantially increased so that the goalof promoting recovery efficiency can be achieved; Similarly, thecondensed aqueous solution collected by the cold condensed water pipe 80from the first evaporating vessel 10, second evaporating vessel 20 andthird evaporating vessel 30 can also be recovered for reusing in therinse process of the Lyocell fiber production to remove the solvent andimpurities attached on the raw filaments; Thus, the aqueous NMMOsolution 1 generated by the whole Lyocell fiber production process canbe completely recovered to meet the legislation requirements of theenvironmental protection.

In order to prove the enhancement of the recovery efficiency for thesequential multi-stage evaporating system aforesaid, following realembodiment examples and comparative examples are experimented toillustrate the resultant recovery efficiency. The recovery efficiency ofthe NMMO is calculated by the following formula.

${\underset{\_}{RVEff}\mspace{14mu} {of}\mspace{14mu} {NMMO}} = {\frac{{CnAR}\mspace{14mu} \% \times {OQAR}}{{CnBR}\mspace{14mu} \% \times {IQBR}} \times 100\%}$

Where, RVEff denotes to “recovery efficiency”;

CnBR denotes to “Concentration before Recovery”;

CnAR denotes to “Concentration after Recovery”;

IQBR denotes to “Inlet Quantity before Recovery”; and

OQAR denotes to “Outlet Quantity after Recovery”.

EMBODIMENT EXAMPLE 1

Please refer to FIG. 3, which is a schematic diagram of a preferredexemplary embodiment in a bleach step showing the addition of activatedcarbon powder therein in association with a disposition of an agitationblower for the present invention. After previous processes of bleach(step 101) and filtration (step 102), the collected aqueous NMMOsolution 1 of the Lyocell fiber will contain NMMO in 3.92 wt % weightpercentage. Then, the collected aqueous NMMO solution 1 is input intothe first evaporating vessel 10 via input pipe 50 by orderly passingthird steam tank 31, second steam tank 21 and first steam tank 11 aswell as heat exchanger 6 and input pump 51. Because the aqueous NMMOsolution 1 has orderly passed third steam tank 31, second steam tank 21and first steam tank 11, the steam evaporated by the recovered aqueoussolution at the outlets of the first evaporating vessel 10, secondevaporating vessel 20 and third evaporating vessel 30 can be fed back tothe first evaporating vessel 10 as supplementary steam source via thesteam recovering pipe 42 after it has been orderly processed by thefirst steam tank 11, second steam tank 21 and third steam tank 31 aswell as separating tank 40 and steam compressor 41 so that the aqueousNMMO solution 1 can be heated up to the desired temperature before itenters the first evaporating vessel 10.

For the first dehydration in the first evaporating vessel 10, theprocess is performed by following operating parameters.

Heating medium: By inputting steam generated by a steam boiler (notshown) to serve as primary steam source into the first evaporatingvessel 10, it evaporates the aqueous NMMO solution 1 therein to operatefirst dehydration.

Outlet Concentration: 12 wt % for the recovered aqueous NMMO solution 1of the first evaporating vessel 10 (measured by first concentrationmeter 15).

Degree of Vacuum: 600 mmHg (by acting first vacuum pump 13 on the firststeam tank 11).

Operating Temperature: 70.0-73.0° C. (70.0-73.0 degree of Celsius).

For the second dehydration in the second evaporating vessel 20, theprocess is performed by following operating parameters.

Heating medium: By inputting steam evaporated by aqueous NMMO solution 1of the first evaporating vessel 10 to serve as vaporized steam sourceinto the second evaporating vessel 20 orderly via first steam inlet pipeC1, first steam tank 11 and first steam outlet pipe 12, it evaporatesthe aqueous NMMO solution 1 therein to operate second dehydration.

Outlet Concentration: 28 wt % for the recovered aqueous NMMO solution 1of the second evaporating vessel 20 (measured by second concentrationmeter 25).

Degree of Vacuum: 630 mmHg (by acting second vacuum pump 23 on thesecond steam tank 21).

Operating Temperature: 61.0-62.5° C. (61.0-62.5 degree of Celsius).

For the third dehydration in the third evaporating vessel 30, theprocess is performed by following operating parameters.

Heating medium: By inputting steam evaporated by aqueous NMMO solution 1of the second evaporating vessel 20 to serve as vaporized steam sourceinto the third evaporating vessel 30 orderly via second steam inlet pipeC2, second steam tank 21 and second steam outlet pipe 22, it evaporatesthe aqueous NMMO solution 1 therein to operate third dehydration.

Outlet Concentration: 50.05 wt % for the recovered aqueous NMMO solution1 of the third evaporating vessel 30 (measured by third concentrationmeter 35).

Degree of Vacuum: 650 mmHg (by acting third vacuum pump 33 on the thirdsteam tank 31).

Operating Temperature: 51.8-52.2° C. (51.8-52.2 degree of Celsius).

Besides, because the aqueous NMMO solution 1 has orderly passed thirdsteam tank 31, second steam tank 21 and first steam tank 11, the steamevaporated by the recovered aqueous solution at the outlets of the firstevaporating vessel 10, second evaporating vessel 20 and thirdevaporating vessel 30 can be fed back to the first evaporating vessel 10as supplementary steam source via the steam recovering pipe 42 after ithas been orderly processed by the first steam tank 11, second steam tank21 and third steam tank 31 as well as separating tank 40 and steamcompressor 41 so that the aqueous NMMO solution 1 can be heated up tothe desired temperature before it enters the first evaporating vessel10. Thus, the supplementary steam source recovered from the thirdevaporating vessel 30 can achieve effect in preheating the aqueous NMMOsolution 1 in the first evaporating vessel 10.

All foregoing operating parameters of the preferred exemplary embodimentexample I are collected and tabulated in “Extracted embodiment example 1from Table-1” shown as below.

Moreover, the outlet quantity after recovery from the third evaporatingvessel 30 is 925.5 ton(s) while the inlet quantity before recovery tothe first evaporating vessel 10 is 11835 ton(s).

In summary, the following key operating parameters are recapitulated.

The Inlet Quantity before Recovery is 11,835 ton(s).

The Outlet Quantity after Recovery is 925.5 ton(s).

The Concentration before Recovery is 3.92%.

The Concentration after Recovery is 50.05%.

According to foregoing key operating parameters, the recovery efficiency(RVEff) is calculated by the following formula predetermined.

${\underset{\_}{RVEff}\mspace{14mu} {of}\mspace{14mu} {NMMO}} = {\frac{{CnAR}\mspace{14mu} \% \times {OQAR}}{{CnBR}\mspace{14mu} \% \times {IQBR}} \times 100\%}$$\frac{{50.05\% \times 925.5} = {46321.3\%}}{{3.92\% \times 11835} = {46393.2\%}} \times 100\%$${\underset{\_}{RVEff}\mspace{14mu} {of}\mspace{14mu} {NMMO}} = {99.8\%}$

Basing on the values for related parameters in the “Extracted embodimentexample 1 from Table-2” shown as below, the recovery efficiency (RVEff)of NMMO obtained is 99.8%.

[Extracted embodiment example 1 from Table-1] 1st dehydration 2nddehydration 3rd dehydration 1st evaporating 2nd evaporating 3rdevaporating vessel vessel vessel H.M. primary steam vaporized steam fromvaporized steam from from steam boiler 1st steam tank 11 2nd steam tank21 E.E. OC DV OT OC DV OT OC DV OT wt mm ° C. wt mm ° C. wt mm ° C. % Hg% Hg % Hg Em. 12 600 70.0- 28 630 61.0- 50.05 650 51.8- 1 73.0 62.5 52.2R.S. supplementary CtAS with CdAS without steam NMMO solvent NMMOsolvent From steam recovering solution recovering cold condensed pipe 42pipes 14, 24 and 34 water pipe 80 To 1st evaporating storage tank 70Rinse step vessel 10 Denotation H.M. denotes to “Heating Media”. E.E.denotes to “Embodiment Example”. Em denotes to “Example”. OC denotes to“Outlet Concentration”. DV denotes to “Degree of Vacuum”. OT denotes to“Operating Temperature”. R.S. denotes to “Recovered Substance”. CtASdenotes to “Concentrated Aqueous Solution”. CdAS denotes to “CondensedAqueous Solution”.

[Extracted embodiment example 1 from Table-2] Embodiment IQBR CnBR OQARCnAR RVEff Example ton(s) wt % ton(s) wt % % Example 1 11,835 3.92 925.550.05 99.80 Denotation IQBR denotes to “Inlet Quantity before Recovery”.CnBR denotes to “Concentration before Recovery”. OQAR denotes to “OutletQuantity after Recovery”. CnAR denotes to “Concentration afterRecovery”. RVEff denotes to “Recovery Efficiency”.

EMBODIMENT EXAMPLE 2-9

The processing procedure of bleach step for preferred exemplaryembodiment example 2-9 in the present invention is the same as that forpreferred exemplary embodiment example 1 but with following differences.

Outlet Concentration: In range of 10-20 wt % for the recovered aqueousNMMO solution 1 of the first evaporating vessel 10 (measured by firstconcentration meter 15).

Outlet Concentration: In range of 22-38 wt % for the recovered aqueousNMMO solution 1 of the second evaporating vessel 20 (measured by secondconcentration meter 25).

For other operating parameters such as outlet concentration (OC), degreeof vacuum (DV) and operating temperature (OT), they are listed in theTable-1. Moreover, for other key operating parameters such as inletquantity before recovery (IQBR), outlet quantity after recovery (OQAR),concentration before recovery (CnBR) and concentration after recovery(CnAR), they are also listed in the Table-2. Thus, the recoveryefficiency (RVEff) is calculated by the same formula aforesaid inaccordance with foregoing key operating parameters listed in theTable-2.

COMPARATIVE EXAMPLE 1-9

The processing procedure of bleach step for preferred exemplarycomparative example 1-9 in the present invention is the same as that forpreferred exemplary embodiment example 1 but with following differences.

Outlet Concentration: In range of 10-20 wt % for the recovered aqueousNMMO solution 1 of the first evaporating vessel 10 (measured by firstconcentration meter 15).

Outlet Concentration: In range of 22-38 wt % for the recovered aqueousNMMO solution 1 of the second evaporating vessel 20 (measured by secondconcentration meter 25).

For other operating parameters such as outlet concentration (OC), degreeof vacuum (DV) and operating temperature (OT), they are listed in theTable-1. Moreover, for other key operating parameters such as inletquantity before recovery (IQBR), outlet quantity after recovery (OQAR),concentration before recovery (CnBR) and concentration after recovery(CnAR), they are also listed in the Table-2. Thus, the recoveryefficiency (RVEff) is calculated by the same formula aforesaid inaccordance with foregoing key operating parameters listed in theTable-2.

[Result]:

Referring to Table-2, with processing condition of the OutletConcentration being in range of 10-20 wt % for the recovered aqueousNMMO solution 1 of the first evaporating vessel 10 (measured by firstconcentration meter 15) and the Outlet Concentration: being in range of22-38 wt % for the recovered aqueous NMMO solution 1 of the secondevaporating vessel 20 (measured by second concentration meter 25), eachrecovery efficiency (RVEff) for all the preferred exemplary embodimentexample 1-9 of the present invention listed in upper portion thereof isbetter than that corresponding in all the preferred exemplarycomparative example 1-9 of the present invention listed in lower portionthereof.

For convenience, the Table-1 and Table-2 are listed in other sheet asattached.

Obviously, with processing condition of the Outlet Concentration beingin range of 10-20 wt % for the recovered aqueous NMMO solution 1 of thefirst evaporating vessel 10 (measured by first concentration meter 15)and the Outlet Concentration: being in range of 22-38 wt % for therecovered aqueous NMMO solution 1 of the second evaporating vessel 20(measured by second concentration meter 25) as well as feeding the steamevaporated by the recovered aqueous solution at the outlet of the thirdevaporating vessel 30 back to the first evaporating vessel 10 assupplementary steam source via the steam recovering pipe 42 after it hasbeen orderly processed by the third steam tank 31, separating tank 40and steam compressor 41, the overall recovered quantity of theconcentrated aqueous NMMO solution under the same consumed quantity ofthe primary steam source can be substantially increased so that the goalof promoting recovery efficiency can be achieved.

The refinement in step 104 means for purifying the aqueous NMMO solution1.

Before the concentrated aqueous NMMO solution is subjected to therefinement step, a bit of residual N-methylmorpholine (NMM) that arisesfrom decomposition of NMMO, which is caused by heating during thedissolution step in the production of Lyocell fiber. In this preferredexemplary embodiment, the quantity of residual NMM in the concentratedaqueous NMMO solution is in range of 0.1-0.3 wt %. To perform therefinement step here, an oxidizer (namely oxidizing agent) is added intothe concentrated aqueous NMMO solution processed by previous refinementprocess (step 103) so that the residual N-methylmorpholine (NMM) isoxidized into N-methylmorpholine-N-oxide (NMMO) via oxidation reactionby the oxidizer under reaction temperature being 80±2° C.(80.+/−0.2.degree of Celsius). After the oxidation reaction aforesaid,some residual oxidizer becomes redundant impurity, which should becompletely removed anyhow. Accordingly, a reducer (namely neutralizingagent) is added into the concentrated aqueous NMMO solution processed byprevious oxidation reaction process aforesaid to neutralize the residualoxidizer via reduction reaction by the reducer to a quantity in rangeless than 0.06 wt % so that a recovered aqueous NMMO solution of highpurity is obtained. Wherein, the oxidizer applied is H₂O₂ (hydrogenperoxide), and the reducer applied is N₂H₄H₂O (hydrazine hydrate) inthis preferred exemplary embodiment. Moreover, the final appliedquantities for the oxidizer and the reducer are decided by the testingresult of the concentrated aqueous NMMO solution processed by foregoingredox reaction (namely reduction reaction and oxidation reaction) viapotentiometric titration in this preferred exemplary embodiment.

Though a small quantity of NMM with concentration less than 0.06 wt %already contained in a fresh NMMO solvent, some more NMM may be createddue to heating decomposition from a small portion of fresh NMMO solventduring production of Lyocell fiber. If the concentrated aqueous NMMOsolution is not subjected to the refinement step here for processingforegoing overall NMM, the concentrated aqueous NMMO solution has a badability in dissolving cellulose when it is recovered for reuse so thatit may not only easily cause adverse affect for spinning efficiency suchas obstruction in spinneret orifices and breakages of spinning filamentsand the like during the spin step in production of Lyocell fiber butalso incur deteriorating physical properties for the fabrics thereofsuch as declined tenacity. Accordingly, in order to avoid the quality ofLyocell fiber fabrics from being detrimentally affected by the recoveredaqueous NMMO solution in the present invention so as to have asatisfactory resultant quality thereof, the refinement step becomescritically imperative to oxidize existing residual NMM contained in theconcentrated aqueous NMMO solution into NMMO other than the achievementfor the preset concentration of the concentrated aqueous NMMO solutionso that not only the purity of the NMMO is enhanced but also the wastageof the NMMO is reduced.

Besides, in this preferred exemplary embodiment, the reason forpresetting reaction temperature of the N-methylmorpholine (NMM) to 80±2°C. (80.+/−0.2.degree of Celsius) is on the basis of followingconsiderations. If the reaction temperature is excessively higher thanthe presetting reaction temperature, NMM and oxidizer H₂O₂ (hydrogenperoxide) in the concentrated aqueous NMMO solution can be easilydecomposed and volatilized so that energy is wasted incurred by theviolent status of the redox reaction. Contrarily, if the reactiontemperature is excessively lower than the presetting reactiontemperature, the refinement efficiency is decrease incurred by theinvalid status of the redox reaction.

Wherein, the chemical reaction equation for the oxidation reaction ofNMM by the oxidizer H₂O₂ (hydrogen peroxide) in the concentrated aqueousNMMO solution is shown as follows:

Reductant Oxidation Product C₅H₁₁NO + H₂O₂ → C₅H₁₁NO₂ + H₂O Theoxidation reaction takes place under presetting reaction temperature of80 ± 2° C. (80 .+/−. 2 .degree of Celsius) for 2 hours.

Where, the “C₅H₁₁NO” denotes the molecular formula for theN-methylmorpholine (NMM); the “C₅H₁₁NO₂” denotes the molecular formulafor the N-methylmorpholine-N-oxide (NMMO); and the “H₂O₂” denotes themolecular formula for the oxidizer (hydrogen peroxide).

Whereas, the chemical reaction equation for the reduction reaction ofresidual oxidizer H₂O₂ (hydrogen peroxide) by the reducer N₂H₄H₂O(hydrazine hydrate) in the concentrated aqueous NMMO solution is shownas follows:

Oxidant Reduction Product N₂H₄•H₂O + 2H₂O → 5H₂O + N₂ The reductionreaction also takes place under presetting reaction temperature of 80 ±2° C. (80 .+/−. 2 .degree of Celsius) for 2 hours.

Where, the “N₂H₄.H₂O” denotes the molecular formula for the reducer(hydrazine hydrate).

By means of foregoing chemical reaction equations, the content of NMM inthe concentrated aqueous NMMO solution can be firstly measured, then therequired adding quantity of oxidizer H₂O₂ (hydrogen peroxide) can beroughly calculated by the reactive molar ratio of NMM to oxidizer H₂O₂(hydrogen peroxide) in the first chemical reaction equation of theoxidation reaction subsequently. However, in order to ensure that almostall of NMM can be completely oxidized, an actual adding quantity ofoxidizer H₂O₂ (hydrogen peroxide) to the concentrated aqueous NMMOsolution is more than the forgoing calculated adding quantity with a bitof extra residual oxidizer H₂O₂ (hydrogen peroxide) remained after theoxidation reaction. Successively, after the oxidation reaction of NMM,the reducer N₂H₄.H₂O (hydrazine hydrate) is added to neutralize theextra residual oxidizer H₂O₂ (hydrogen peroxide). With foregoing twochemical reaction equations, even though adequate quantities of theoxidizer H₂O₂ (hydrogen peroxide) and the reducer N₂H₄.H₂O (hydrazinehydrate) are added to the concentrated aqueous NMMO solution, the finalproducts still include H₂O and N₂ other than the NMMO after havingforegoing two chemical reaction equations finished. Wherein, N₂ can bedirectly dispersed into air, and H₂O becomes a useful portion of therecovered aqueous NMMO solution. Thus, no impurities and redundant sideproducts are created in the recovered aqueous NMMO solution during therefinement step so that the concentrated aqueous NMMO solution becomesnot only high purity of recovered aqueous NMMO solution and but alsohigh purity of fabrics from Lyocell fiber without any bad effect.

In conclusion of disclosure heretofore, the method of recovering aqueousN-Methylmorpholine-N-Oxide solution used in production of Lyocell fiberof the present invention features following novelties.

-   -    Bleach in step 101 means for decoloring harmful coloration        with contaminate pigment impurities in the aqueous NMMO solution        1 via alternate blow-mixing adsorption mode and static        suspending adsorption mode reiteration by activated carbon        powder 3.    -    Filtration in step 102 means for purifying the activated        carbon powder 3 and impurities from the aqueous NMMO solution 1        by means of two filtering stages of first coarse filtering stage        and successive second fine filtering stage (ultrafiltration UF)        being orderly adopted.    -    Concentration in step 103 means for intensifying the aqueous        NMMO solution 1 to obtain a desired condensed aqueous solution        without NMMO solvent and an expected concentrated aqueous        solution with NMMO solvent respectively by a sequential        multi-stage evaporating system of high-performance.    -    Refinement in step 104 means for purifying the aqueous NMMO        solution 1 with promoting purity of the concentrated aqueous        solution to obtain required recovered aqueous solution by adding        suitable agents in the redox reactions involved so that not only        the aqueous NMMO solution 1 but also massive water can be        recovered and reused as processing materials in the production        of Lyocell fiber.

Therefore, the recovering method of the present invention not only hascompletely recovered massive aqueous NMMO solution 1 to substantiallyreduce wastes discharged to the environment but also has almostrecovered the NMMO solvent in the aqueous NMMO solution 1 to essentiallyreduce material cost in the production of Lyocell fiber. Thus, thefeatures of the present invention not only save processing cost but alsomeet requirements of environment protection.

What is claimed is:
 1. A method of recovering aqueousN-Methylmorpholine-N-Oxide (NMMO) solution used in production of Lyocellfiber comprises steps of bleach (step 1), filtration (step 2),condensation (step 3) and refinement (step 4), wherein: The bleach instep 1 means for decoloring the aqueous NMMO solution: Firstly, theaqueous NMMO solution to be recovered is loaded into a bleaching tank,and activated carbon powder featuring with good adsorbability andsuspendability is added into the aqueous NMMO solution therein;secondly, the activated carbon powder and the aqueous NMMO solution aremixed together by using an agitation blower; and finally, the agitationblower is intermittently energized so that an alternate blow-mixingadsorption mode and static suspending adsorption mode reiterates to haveactivated carbon powder fully contacted with the aqueous NMMO solutionthoroughly in an energy-efficient manner; thereby, the adsorptionefficiency of the activated carbon powder is essentially increased;preferably, the added dosage of the activated carbon powder is in rangeof 0.05 wt %-0.10 wt % on the basis of total weight for the aqueous NMMOsolution; wherein, the blow-mixing adsorption mode means for blowingambient air into the bleaching tank by impeller rotation of theagitation blower to have activated carbon powder fully contacted withthe aqueous NMMO solution thoroughly to facilitate the speed of thebleach process while the static suspending adsorption mode means forkeeping the aqueous NMMO solution in stationary manner to let theaqueous NMMO solution precipitate automatically to save related energy;moreover, by means of a timer switch acting on the agitation blower, theblow-mixing adsorption mode and static suspending adsorption mode areintermittently alternated in reiterative fashion so that overall bleachefficiency is substantially increased; a duration ratio for theblow-mixing adsorption mode and static suspending adsorption mode is inrange of 1:3-1:6; besides, total time in the bleach process of theaqueous NMMO solution is not longer than 8 hours; thus, by means ofblow-mixing adsorption mode, not only the activated carbon powder can befully contacted with the aqueous NMMO solution thoroughly but also thespeed of the bleach process can be facilitated while by means of staticsuspending adsorption mode, not only the processing energy can beessentially saved but also the efficiency of the bleach process can beenhanced; The filtration in step 2 means for purifying the aqueous NMMOsolution: Two filtering stages of first coarse filtering stage andsuccessive second fine filtering stage (ultrafiltration UF) are orderlyadopted so as to remove the activated carbon powder and impurities fromthe aqueous NMMO solution 1, which has been decolored in previous bleachprocess; for the first coarse filtering stage, a filter cartridge havingfiltering material with pore size in range of 1 μm-100 μm (1 .mu-100.mu.m) is used so that the activated carbon powder and the impurities oflarge particle size appeared in previous bleach process can be removed;for the second fine filtering stage (ultrafiltration UF), a filtermaterial with pore size in range of 0.01 μm-1 μm (0.01 .mu.m-1 .mu.m) isused so that the tiny impurities of small particle size can be removed;wherein, the cartridge filter used in the first coarse filtering stage,over the surface thereof is beforehand pre-coated a filter aid, which ismade from mixture of diatomaceous earth and cellulose with weight ratioof the diatomaceous earth to the cellulose is 4:1 preferably so that itprevents the activated carbon powder from accumulating on the surfacethereof in hindering the filtering speed; besides, the filter aid isalso added into the bleached aqueous NMMO solution with quantity inrange of 0.03-0.05 wt % to increase the valid filtering area; wherein,some filtering dregs, which contain residual filter aid and a largerquantity of residual activated carbon powder mostly accumulated on thesurface portion, are created after the first coarse filtering stage;after the filtering dregs is scraped off, the residual filter aidtherein can be recovered and reused in the first coarse filtering stagestep; The concentration in step 3 means for intensifying the aqueousNMMO solution: A sequential multi-stage evaporating system is adopted soas to intensify the aqueous NMMO solution, which has been purified inprevious filtration process so that a condensed aqueous solution withoutNMMO solvent and a concentrated aqueous solution with NMMO solvent arerespectively obtained; the sequential multi-stage evaporating systemmainly comprises a first evaporating vessel with a first steam tank, asecond evaporating vessel with a second steam tank and a thirdevaporating vessel with a third steam tank, wherein: the firstevaporating vessel and the first steam tank are connected by a firststeam inlet pipe while the first steam tank and the second evaporatingvessel are connected by a first steam outlet pipe such that the firststeam tank is connected to a first vacuum pump; moreover, the firstevaporating vessel and second evaporating vessel are connected by afirst solution recovering pipe, on which a first concentration meter anda first suction pump are disposed respectively; the second evaporatingvessel and the second steam tank are connected by a second steam inletpipe while the second steam tank and the third evaporating vessel areconnected by a second steam outlet pipe such that the second steam tankis connected to a second vacuum pump; moreover, the second evaporatingvessel and third evaporating vessel are connected by a second solutionrecovering pip, on which a second concentration meter and a secondsuction pump are disposed respectively; the third evaporating vessel andthe third steam tank are connected by a third steam inlet pipe while thethird steam tank is connected to a third vacuum pump and a third steamoutlet pipe, which is further connected to a separating tank for steamand aqueous solution; moreover, the third evaporating vessel and asecond suction pump are connected by a third solution recovering pipe,on which a third concentration meter and a third suction pump aredisposed respectively; wherein, the aqueous solution outlet of theseparating tank is connected to the third solution recovering pipe whilethe steam outlet of the separating tank is connected to a mechanicalsteam compressor, which is further connected to a first evaporatingvessel via a steam recovering pipe; for the first evaporating vessel,other than the recovered steam being fed via the steam recovering pipe,a primary steam from a steam boiler is supplied via an input pipe forsteam, and an aqueous NMMO solution, which has been purified in previousfiltration process, is also supplied by an input pipe for aqueous NMMOsolution, which orderly passes through the third steam tank, secondsteam tank and first evaporating vessel as well as a heat exchanger anda input pump for aqueous NMMO solution; by controlling the concentrationof the recovered aqueous solution at the outlet of the first evaporatingvessel in range of 10-20 wt % and the concentration of the recoveredaqueous solution at the outlet of the second evaporating vessel in rangeof 22-38 wt % as well as feeding the steam evaporated by the recoveredaqueous solution at the outlet of the third evaporating vessel back tothe first evaporating vessel as supplementary steam source via the steamrecovering pipe after it has been orderly processed by the third steamtank, separating tank and steam compressor, the overall recoveredquantity of the concentrated aqueous NMMO solution under the sameconsumed quantity of the primary steam source can be substantiallyincreased so that the goal of promoting recovery efficiency can beachieved; similarly, the condensed aqueous solution collected by thecold condensed water pipe from the first evaporating vessel, secondevaporating vessel and third evaporating vessel can also be recoveredfor reusing in the rinse process of the Lyocell fiber production toremove the solvent and impurities attached on the raw filaments; and Therefinement in step 4 means for purifying the aqueous NMMO solution:Before the concentrated aqueous NMMO solution is subjected to therefinement step, a bit of residual N-methylmorpholine (NMM) that arisesfrom decomposition of NMMO, which is caused by heating during thedissolution step in the production of Lyocell fiber; the quantity ofresidual NMM in the concentrated aqueous NMMO solution is in range of0.1-0.3 wt %; to perform the refinement step here, an oxidizer (namelyoxidizing agent) is added into the concentrated aqueous NMMO solutionprocessed by previous refinement process (step 3) so that the residualN-methylmorpholine (NMM) is oxidized into N-methylmorpholine-N-oxide(NMMO) via oxidation reaction by the oxidizer under reaction temperaturebeing 80±2° C. (80.+/−0.2.degree of Celsius); after the oxidationreaction aforesaid, some residual oxidizer becomes redundant impurity,which should be completely removed anyhow; accordingly, a reducer(namely neutralizing agent) is added into the concentrated aqueous NMMOsolution processed by previous oxidation reaction process aforesaid toneutralize the residual oxidizer via reduction reaction by the reducerto an quantity in range less than 0.06 wt % so that a recovered aqueousNMMO solution of high purity is obtained; wherein, the oxidizer appliedis H₂O₂ (hydrogen peroxide), and the reducer applied is N₂H₄H₂O(hydrazine hydrate); moreover, the final applied quantities for theoxidizer and the reducer are decided by the testing result of theconcentrated aqueous NMMO solution processed by foregoing redox reaction(namely reduction reaction and oxidation reaction) via potentiometrictitration.